This grant seeks to understand the role of Group 2 innate lymphoid cells (ILC2s) in sustaining an altered epithelial interface that is crucial to the establishment of an aberrant remodeled niche that contributes to the persistence of allergic pathology. The proposal is underpinned by publications supported in the prior grant, demonstrating (1) a key role for tissue-elaborated epithelial cytokines in the activation of tissue type 2 immune cells, including ILC2s; and (2) discovery of a feed-forward epithelial-ILC2 circuit in the intestines that drives goblet cell hyperplasia and tissue hyperplasia, providing the impetus for uncovering similar circuitry in the respiratory system. As shown by colleagues in Project 3, severe, drug-recalcitrant, asthma is populated disproportionately with patients with relatively ?fixed? airway abnormalities associated with persistent mucus plugs; a significant proportion of individuals also have recurrent nasal polyposis. Together with our discoveries in model animal systems, our over-arching hypothesis is that dysregulated epithelial-ILC2 circuits in the respiratory tract, including nasopharynx (NP), trachea and airways, underlie the presence of persistent niches where allergic pathology is sustained. Using novel genetic tools in mice that permit exquisite dissection of such circuits, we propose 3 Specific Aims that align within the overall goals of the PPG. First, we will define the ILC2 landscape in mouse skin, NP, trachea and lung, where we have uncovered unsuspected diversity that has already been extended to human studies by others in the PPG. Second, we will define the tuft cell landscape in NP and trachea, since the critical role for this enigmatic epithelial cell was uncovered in our intestinal studies. We have demonstrated tuft cell hyperplasia in human allergic polyps. Third, we will use a mouse model of skin inflammation followed by lung allergen challenge to analyze the development of the epithelial-ILC2 circuit from skin to NP to lung, and the role of these respective cells. These findings will be applied to human tissues collected in Core B of this PPG and analyzed in Core C of this PPG to generate discovery in patients with severe asthma. Together, this PPG will uncover dysfunctional cellular networks and niches that sustain allergic immunopathology in the lung.